Issue with sharing cache across jobs

[aloysbaillet]

Hi,

I am trying to reuse a "local" cache across multiple CI jobs without using a registry. The main reason to avoid the registry is to nicely associate a CI run with the generated artifact, and not adding more images on the registry when the CI system already supports artifact tied to the builds.

I made an example repo with 2 images: imageA builds python from source using ccache https://github.com/aloysbaillet/buildx-testing/blob/master/imageA/Dockerfile and the second image just uses the first image. The build commands are in https://github.com/aloysbaillet/buildx-testing/blob/master/.github/workflows/dockerimage.yml but here's a summary:

build A (uses a previous local cache and saves a new cache for future runs)

docker buildx build \
          . \
          -f imageA/Dockerfile \
          -t aloysbaillet/buildx-testing-image-a:0 \
          --cache-from=type=local,src=docker-cache \
          --cache-to=type=local,mode=max,dest=docker-cache-a \
          --load

build B (trying to use cache A to avoid pulling the non-existing image aloysbaillet/buildx-testing-image-a:0)

docker buildx build \
          . \
          -f imageB/Dockerfile \
          -t aloysbaillet/buildx-testing-image-b:0 \
          --cache-from=type=local,src=docker-cache-a \
          --cache-to=type=local,mode=max,dest=docker-cache-b \
          --load

Here is the sequence of builds:
https://github.com/aloysbaillet/buildx-testing/runs/241659655

The main issue I'm facing is how to make job B believe that aloysbaillet/buildx-testing-image-a:0 is a valid docker image. Running docker load is ignored by buildx when using docker-container driver which is necessary for --cache-from and --cache-to to function.
Is there a way to populate the buildkit image from docker? I thought using the cache from job A would have been enough as the image A is tagged and is in cache A...
N.B. it would seem obvious to use a multi-stage job for images A and B, unfortunately in my real-world scenario image A takes around 2 hours to build, and one of my image Bs takes around 7 hours, which is timing out my free CI system... Hence the need to split the jobs and use ccache.

Thanks in advance for any help!

Cheers,

Aloys

[FernandoMiguel]

@tonistiigi ^

[tonistiigi]

N.B. it would seem obvious to use a multi-stage job for images A and B, unfortunately in my real-world scenario image A takes around 2 hours to build, and one of my image Bs takes around 7 hours, which is timing out my free CI system... Hence the need to split the jobs and use ccache.

You can still use a multi-stage Dockerfile with two separate runs where first time you build the a target and second time the b target(that depends on a). Instead of doing FROM a you just depend on the same parts of Dockerfile that you built before, therefore you get cache for it and it doesn't need to be rebuilt.

Eg.

FROM ubuntu AS stage-a
...

FROM stage-a
...

build --target=stage-a --cache-to=type=local,mode=max,dest=docker-cache-a
build --cache-from=type=local,src=docker-cache-a

For example, we use same pattern in buildkit CI parallelization. The first task builds integration-tests-base stage and exports the cache for it https://github.com/moby/buildkit/blob/master/hack/build_ci_first_pass#L35 . Other tasks that now run in parallel, build integration-tests stage that is on top of base stage with importing cache https://github.com/moby/buildkit/blob/master/hack/dockerfiles/test.buildkit.Dockerfile#L231

[aloysbaillet]

Thank you very much Tonis!
This is a great idea indeed, and I will try it as soon as possible. One thing to note is that I will end up with a single very large Dockerfile ( I have 7 at around 50-100 lines, so roughly 500 lines). This also means that I have to share bits of cache that are not relevant between stages such as the ccache content of upstream builds.
I also noticed that the content of the mounted cache doesn't seem to be included in the --cache-to in local mode, is that a known issue?
Cheers,

Aloys

[tonistiigi]

Cache mounts is a different concept from the instruction cache. They are local persistent directories that don't have a deterministic state. Theoretically, you can use the build itself to read files into or out of these directories but I don't think it would give you performance increase. The point of the instruction cache is to determine that the sources are still valid for build and skip over the instructions, while cache mounts provide an incremental boost while the run command is running.

For the big Dockerfiles, there has been discussions for supporting "include" to split files apart but nobody has made a prototype yet.

[aloysbaillet]

Thanks Tonis.
Is there any way to persist the mounted cache folders across multiple CI jobs? The extra boost means that builds where the exact instruction cache is not met would go from 3-7 hours down to 10 minutes so still very very valuable. I was looking at copying the volume used by the buildx docker container as a way to persist the whole cache but that seems excessive and I'm unsure if buildx will let me do this?

[tonistiigi]

Is there any way to persist the mounted cache folders across multiple CI jobs?

Only way to do this atm is with the build request itself. Eg. you can build a stage that loads files into a cache mount or a stage that just returns the files in the cache mount. I don't recommend doing this though unless you can clearly measure that it improves your performance.

[aloysbaillet]

Thanks again Tonis!
I'm was assuming that multiple RUN --mount=type=cache,target=/tmp/ccache commands in a given Dockerfile would not share the actual same /tmp/ccache folder, but your answer seems to indicate that the target path is the only identifier to reuse this cache mount? I've been wondering about the exact behaviour of the cache mount...
I'd be happy to create a PR with added documentation on how the cache mount gets reused/invalidated if I get to find out :-)

[FernandoMiguel]

@aloysbaillet for the same named target, the cache mount will be shared between jobs in the same builder.
You can use locks to prevent concurrent runs if that causes issues, but it will impact your performance ofc.

For instruction cache, you can push the intermediary layers to a network close registry and access that from multiple builders

[aloysbaillet]

Thanks Fernando! That's great info, and I updated my test repo with all the useful knowledge gathered here.
With all this I believe I have enough to really benefit from buildx.
That said, I would still really like to know if there is any way to do the equivalent of docker load into a buildx container, it would be a nice closure for this issue :-)

[aloysbaillet]

So I went back to my test project and tried to use both instruction cache and mounted cache as fallback, but I can't find a way to use these both at the same time!

I'm stuck between using instruction cache (really fast when every character of the RUN command has not changed, but needs complete rebuild when anything there changes) and the fallback of mounted cache which works really well to speed up builds when instruction cache is not matching (in my case a 3 hour build becomes a 10m build).

The main problem with trying to use both caches is that I need to somehow inject the content of the mounted cache into the build without invalidating the instruction cache (and ideally without requiring a rsync server as explained there: http://erouault.blogspot.com/2019/05/incremental-docker-builds-using-ccache.html ).

Using a bind mount to expose the previous ccache content cannot work as the first build will have an empty cache to mount and that is recorded in the instruction cache, and the second build will use the first build's cache, but find a non-empty ccache which will invalidate the instruction cache from the first build.

See this file for an example: https://github.com/aloysbaillet/buildx-testing/blob/master/Dockerfile#L15

FROM n0madic/alpine-gcc:8.3.0 as buildx-testing-image-a-builder

RUN --mount=type=cache,target=/tmp/ccache \
    --mount=type=cache,target=/tmp/downloads \
    --mount=type=bind,source=ccache,target=/tmp/ccache_from \
    export CCACHE_DIR=/tmp/ccache && \
    export DOWNLOADS_DIR=/tmp/downloads && \
    if [ -f /tmp/ccache_from/ccache.tar.gz ] ; then cd /tmp/ccache && tar xf /tmp/ccache_from/ccache.tar.gz && cd - ; fi && \
    if [ ! -f $DOWNLOADS_DIR/Python-3.7.3.tgz ] ; then curl --location https://www.python.org/ftp/python/3.7.3/Python-3.7.3.tgz -o $DOWNLOADS_DIR/Python-3.7.3.tgz ; fi && \
    tar xf $DOWNLOADS_DIR/Python-3.7.3.tgz && \
    cd Python-3.7.3 && \
    ./configure \
        --prefix=/usr/local \
        --enable-shared && \
    make -j4 && \
    make install && \
    ccache --show-stats && \
    tar cfz /tmp/ccache.tar.gz /tmp/ccache

FROM scratch as buildx-testing-image-a-ccache

COPY --from=buildx-testing-image-a-builder /tmp/ccache.tar.gz /ccache/ccache.tar.gz

which is used there: https://github.com/aloysbaillet/buildx-testing/blob/master/.github/workflows/dockerimage.yml#L30

tar xf ccache/ccache.tar.gz

# buildx-testing-image-a
docker buildx build \
  . \
  -t aloysbaillet/buildx-testing-image-a:0 \
  --target=buildx-testing-image-a \
  --cache-from=type=local,src=docker-cache \
  --cache-to=type=local,mode=max,dest=docker-cache-a \
  --load

# buildx-testing-image-a-ccache
docker buildx build \
  . \
  --target=buildx-testing-image-a-ccache \
  --platform=local \
  -o .

It really feels like to make this work we would need a new source flag in the mount command to be used like this: --mount=type=cache,target=/tmp/downloads,source=ccache
and it would load the content of the source as the cache initial content.

What do you think?

[FernandoMiguel]

Do keep in mind that github nodes currently don't support any form of cache and each job runs from a new node, so any host cache is lost between runs and jobs

[aloysbaillet]

Indeed, I'm actually maintaining a set of docker images that get built on Azure Pipelines (which has caching available in preview, I believe caching is coming across to GitHub actions very soon...) so I'm emulating this feature by doing a curl of previous build's artifact.

[aloysbaillet]

After some introspection into the buildx container I found this hack that properly moves the whole buildkit cache between nodes:

backup:

        docker buildx create --name cacheable --use
        docker buildx build ... # no --cache-to or --cache-from
        docker run --rm \
          --volumes-from buildx_buildkit_cacheable0 \
          -v $(pwd)/buildkit-cache-a:/backup \
          alpine /bin/sh -c "cd / && tar cf /backup/backup.tar.gz /var/lib/buildkit"

restore:

        docker buildx create --name cacheable --use
        docker buildx inspect --bootstrap
        docker buildx stop
        docker run --rm \
          --volumes-from buildx_buildkit_cacheable0 \
          -v $(pwd)/buildkit-cache-a:/backup \
          alpine /bin/sh -c "cd / && tar xf /backup/backup.tar.gz"

Obviously this is a bit brittle as it assumes the naming convention of the docker container created by buildx create and the location of the buildkit data in the volume...
But it properly restores the cache as if the next job was happening on the same machine as the first one!

[FernandoMiguel]

you can create cache images to disk, if you dont want to use a registry

[aloysbaillet]

Unfortunately --cache-to does not save the mounted cache to disk, that's the main reason I opened this issue...

[FernandoMiguel]

you mean host cache?

[tonistiigi]

It really feels like to make this work we would need a new source flag in the mount command to be used like this: --mount=type=cache,target=/tmp/downloads,source=ccache
and it would load the content of the source as the cache initial content.

Cache mounts support from=basestage. Is this what you are looking for? https://github.com/moby/buildkit/blob/v0.6.2/frontend/dockerfile/docs/experimental.md#run---mounttypecache

[aloysbaillet]

Thanks Tonis, I don't think this from=basestage helps as I can't find any official way to inject this cache data into any build stage without invalidating the instruction cache. The only way I can think of is to somehow download the content of the cache from a locally running file server which is cumbersome.

"mounted cache": the content of the cache defined by --mount=type=cache, keyed by id (which defaults to target):

• content will be updated by many runs of the build command on a single builder instance.
• cannot be pre-populated from the outside of the build

"instruction cache": the buildkit image cache, keyed by each RUN line:

• content is immutable
• can be pre-populated using --cache-from

[tonistiigi]

FROM scratch AS cachebase
ADD /mycachebase.tar /

FROM ...
RUN --mount=type=cache,target=/cache,from=cachebase ...

And you only update mycachebase.tar when you are on a fresh node with no local cache. If you have local cache then you don't update it and use whatever is already in your local cache mount.

[aloysbaillet]

But this means that one has to choose in advance between "mounted cache" and "instruction cache" and it is impossible to know in advance which one will be valid.

I need both caches to be available at all times, here's a timeline of build events:

Build 1:

create empty mountedcachebase.tar file
buildx --to-cache=instructioncache :

FROM scratch AS cachebase
ADD /mycachebase.tar /

FROM ... as builder
RUN --mount=type=cache,target=/cache,from=cachebase ...

FROM ...
COPY --from=builder /cache /mountedcache

-> everything builds

Build 2:

get previous mountedcachebase.tar file
buildx --from-cache=instructioncache :

FROM scratch AS cachebase
ADD /mycachebase.tar / # -> this is different from build1 and invalidate instructioncache

FROM ... as builder
RUN --mount=type=cache,target=/cache,from=cachebase ...

FROM ...
COPY --from=builder /cache

-> everything builds again!

If --cache-to and --cache-from also handled mounted cache then this would also be a solution to this problem.

[Lupus]

I'm also struggling with this. Just installed buildx into my Gitlab docker:dind pipeline in a hope that my mount caches will be exported along with layers, but job retry did not show any signs of cache being exported. And then I stumbled across this issue.

Would be really great if we could control inclusion of mount cache into cache export!

[dbackeus]

It's been 3 years since the last activity in this issue. Can anyone confirm if the state of remote sharing of mount cache remains the same or if there has been any new developments / workarounds in this area?

[tonistiigi]

Going to close this. The initial report is about reusing build result as images in another build what has been resolved with https://www.docker.com/blog/dockerfiles-now-support-multiple-build-contexts/

But then it goes to various unrelated cache mount topics, eventually how to transfer /var/lib/buildkit between machines etc.

For mount cache persistence follow moby/buildkit#1512 or experiment with copying cache mounts to instruction cache in #156 (comment)